1. Field of the Invention
The present invention relates to a sheet stacking apparatus which stacks a sheet, a sheet processing apparatus which processes a sheet stacked in the apparatus, and an image forming apparatus having the above apparatuses.
2. Description of Related Art
Conventionally, some of image forming apparatuses which form an image on a sheet have included a sheet processing apparatus which bundles, binds and folds sheets, on which an image is formed in the main body of the apparatus, to make a booklet. The sheet processing apparatus receives sheets one by one into a tray, and bundles them for alignment. The sheets are bound at the vicinity of the center portion, and the center portions are pierced with an extruding member and are pushed into a nip between a pair of folding rollers. The sheet bundle is conveyed with the pair of folding rollers for folding (Refer to Japanese Patent Application Laid-Open No. 2007-076793).
Operations of such a conventional sheet processing apparatus is described based on FIG. 16. As illustrated in FIG. 16, the sheet processing apparatus first aligns a plurality of sheets at a storage guide 803, and, then, the center portion in the conveying direction is bound with a staple. Subsequently, the center portion of the sheet bundle P is pierced with an extruding member 830, and the extruding member 830 is pushed into a nip between a first pair of folding rollers 810a and 810b. The first pair of folding rollers 810a and 810b, fold the sheet bundle, while conveying the bundle for temporal stopping.
Then, the folded portion is nipped between a second pair of folding rollers 812a and 812b, which is different from the first pair of folding rollers 810a and 810b, and the second folding rollers 812a and 812b are moved along a crease of the sheet in the orthogonal direction to the conveying direction for reinforcement processing of a fold portion. Consequently, the sheet bundle becomes a sheet bundle folded in the middle (hereinafter, simply called “fold sheet bundle”). Subsequently, the fold sheet bundle is conveyed and discharged into a fold bundle tray 840.
Recently, the picture-quality level of the image forming apparatus has been improved, and the kinds of sheet on which an image is formed has been also diversified. For example, a surface-finished special sheet such as a piece of coated paper, and a sheet with abroad range of grammage (inelastic thin paper, and flexible thick paper) can be printed.
However, when an inelastic sheet is aligned and stacked, there will be caused a possibility that a preceding sheet P1, which has already been stacked, and a subsequent sheet P2, which will be conveyed, are conveyed together by a conveying roller 804 as illustrated in FIG. 17 to cause buckling, and defective conveying and defective alignment of the subsequent sheet.
Moreover, in a vertical-path alignment configuration in which a sheet is aligned by a processing tray in which the downstream side in the discharging direction is inclined low, there will be a fear that buckling of an inelastic sheet is caused by its own weight to cause defective conveying and defective alignment of the subsequent sheet. Though there will be considered an idea that a stacking portion of a sheet is made almost horizontal in such a way that there will be no influence by its own weight, there will be a problem in which the size of the apparatus in the lateral direction is made large, and the installation area of the apparatus is increased.
Then, the present invention provides a sheet stacking apparatus, a sheet processing apparatus, and an image forming apparatus, by which various kinds of sheets such as inelastic sheets can be processed without defective conveying and defective alignment.